The 22-year-old featured prominently as the Basque side sent the Red Devils crashing out of the Europa League and will reportedly be watched by Sir Alex Ferguson's scouts againManchester United have made Athletic Bilbao midfielder Oscar de Marcos their primary summer transfer window target, according to The Daily Mirror.
The 22-year-old, who helped the Basque side send United out of the Europa League, reportedly has a price tag of €32 million (£26m).
Sir Alex Ferguson will send his top scout to watch De Marcos in action in the first leg of the semi-final of the competition when Bilbao take on Sporting Lisbon on Thursday.
De Marcos scored in both legs of the tie against United and Ferguson is thought to be compiling a full dossier on the player before meeting the release clause written into his contract.
United are not said to be planning many major additions at the end of the current campaign but will look to strengthen the midfield having felt the impact of Paul Scholes' return from retirement.
Young midfielder Davide Petrucci, meanwhile, will be handed a new three-year contract at Old Trafford.
Petrucci, who has captained United's reserve team, had been linked with a move away from Manchester, with reports in Portugal claiming he could be involved in a possible deal for Benfica star Nicolas Gaitan.
The Italian is now expected to agree terms on a new deal and could spend time out on loan next season as he looks to break into the United first team.